The chrominance (chroma) component of conventional broadcast video signals includes, in sequential format, a synchronizing color burst reference signal followed by color image information. The amplitude of the color burst and the ratio of the amplitude of the color burst to the amplitude of the image information are generally fixed by convention. Not infrequently, the magnitude of the color burst (and the image information) of the received signal deviates from the desired level due to faulty broadcast equipment or the transmission medium, etc. To compensate for these deviations and restore the chrominance signal to nominal levels, conventional receivers include automatic chrominance control (ACC) circuits. The ACC circuits compare the burst magnitude to a preset reference and amplify or attenuate the chrominance signal to maintain the burst signal amplitude constant at the desired level.
Due to faulty ACC operation or differential color burst-image information deviations, the ACC circuit may raise the chrominance signal magnitude undesirably high. The effect of this is to reproduce images with over-saturated colors. To compensate for this latter contingency, chroma overload circuitry is provided which monitors the chrominance signal downstream from the ACC circuitry, and attenuates the chrominance signal when its magnitude exceeds a predetermined amplitude.
In conventional analog receivers, the chroma overload function may be implemented with a simple gain controlled amplifier to provide the signal attenuation and a detector and low pass filter to provide detection. The detector is biased to detect the occurrences of the chroma signal exceeding a desirable range and these detections are integrated by the low pass filter to generate a control signal which is applied to the gain controlled amplifier to appropriately reduce the chrominance signal amplitude.
In a digital television receiver the signal is quantized at regular intervals and occurs as binary numbers synchronous with a system clock. This feature allows more sophisticated averaging techniques of chroma overload occurrences. In accordance with the present invention, chroma overload averaging is made a function of both overload density and signal noise.